Your cost per lead doesn't matter. This number does.
Take your average profit per job. Multiply it by your close rate. That's the most you can pay for one estimate request.
Example: $4,000 profit x 25% close rate = $1,000.
Ads cost me $150 an estimate? I'm fine.
A lead source charges $500 and closes 5%? I'm losing money, even though it "works."
I run a contracting business too. I did this math before I ever bought a lead.
What's your average profit per job, and how many estimates does it take to land one?